In order to design/configure a broadband packet radio communication system characterized by the carrier sense multiple access method such as an IEEE802.11a wireless LAN, it is especially important to design the arrangement of stations in such a manner as to enhance the entire capacity as well as ensuring stable throughput over broad areas.
In such station arrangement design, a repeat design for cells with a plurality of frequency channels in a multi-cluster structure is carried out by arranging the cells efficiently with consideration given to the same channel interference, etc. so that radio zones are sequentially extended.
When this method is applied to make the best use of available frequency bands designated by each country, it is not possible to avoid the arrangement of cells in frequency-position relation where two neighboring cells use adjacent frequency channels or next adjacent frequency channels depending on the cell repeat placement.
FIG. 1 is a diagram showing the configuration of the entire system for explaining problems involved in such frequency relation between cells. As can be seen in FIG. 1, a local channel base station (AP: Access Point) 22 secures as a local channel cell a service area 20, while an adjacent channel base station (AP: Access Point) 23 secures as an adjacent channel cell a service area 21.
Terminals 24 and 25 in the vicinity of the cell boundary area between the local channel service area 20 and adjacent channel service area 21 are connected to the local channel base station and adjacent channel base station, respectively. The terminal 24 is communicating with the local channel base station, and the terminal 25 is communicating with the adjacent channel base station. The two terminals 24 and 25 are at a short distance from each other, and far away from the respective base stations.
The terminals 24 and 25 are communicating with the respective base stations in a condition near the minimum sensitivity reception and maximum output transmission. In consequence, there is concern that the terminal 24, which is performing reception through a local channel, becomes incapable of reception due to interference caused by the transmission of the terminal 25 via an adjacent frequency channel at certain time.
In the case where the transmission and reception of the terminals are performed asynchronously without temporal correlation, a transmission and reception interference generator and a sufferer thereof may be reversed. Consequently, in the vicinity of the cell boundary area, the respective cells become dead zones and incommunicable areas between the cells increase, which causes reductions in the total area of communication cells, the coverage areas and capacity of the entire system, and throughput. This may produce the worst result that interference within the system damages the system itself.
In order to design a transmitter-receiver for avoiding that prospect, improvements in adjacent channel leakage power and spectrum mask are desired for the transmitting end, while highly selective design to enhance the tolerance for adjacent channel reception interference is essential for the receiving end.
The radio communication system characterized by the carrier sense multiple access method such as an IEEE802.11a wireless LAN is compatible with high-data-rate broadband. The occupied bandwidth of modulation transmission/reception waves is considerably wide, and the modulation sidelobe is extended. In addition, the capacity is increased by narrowing the frequency channel interval as much as possible to use limited frequency resources effectively. Therefore, the relative merits of the entire radio communication system is determined by how to improve the ability to eliminate high level adjacent channel interference signals. In other words, this is the main technical key point in designing radio circuits.
In the above description, the adjacent channel interference reception problem arises between the terminals, however, the same adjacent channel interference reception problem may arise between the base stations (AP: Access Point).
Conceivably, the problem is caused in the case where two (or more) channel frequency cells are placed in the same service area to increase only the capacity of the base stations by a factor of two (or more). In this case, two (or more) base stations are placed in close proximity to each other in about the same service area, and therefore, it is not possible to avoid the arrangement of cells in frequency-position relation where the base stations use adjacent frequency channels or next adjacent frequency channels.
In such instances, as is obvious, adjacent channel interference occurs between terminals respectively belonging to the base stations depending on their positions, and further, the adjacent channel interference problem always arises between the base stations depending on a positional relation according to the arrangement of the base stations. Therefore, descriptions of the block configuration and operation of a receiver given hereinafter will apply to a receiver of the terminal and that of the base station.
FIG. 2 (prior art 1) and FIG. 3 (prior art 2) are diagrams each showing the block configuration of a conventional receiver used in a broadband packet radio communication system characterized by the carrier sense multiple access method. As shown in FIG. 2, in the configuration of the receiver according to the prior art 1, a radio frequency signal is received by an antenna 1, and sent to a low noise amplifier (LNA) 3 via a selector switch 2 to be amplified. The amplified signal and the output of a local oscillator (LO) 5 are input in a MIXER 4 to perform frequency conversion.
For the nth-order IF signal converted using such superheterodyne method, digital synchronous detection process is performed after IF sampling, or A/D (Analog/Digital) conversion process is performed after I/Q separation using analog quasi-synchronous detection. In this case, an adjacent channel interference wave is suppressed by providing the nth-order IF section with a band fixed nth-order IF bandpass filter (BPF) 6 having a wideband characteristic (passband) equal to or wider than one-channel modulation occupied band width so as to secure the channel selectivity characteristic.
The output of the BPF 6 is connected to a received signal strength indicator (RSSI) 8 for carrier sensing the preamble of a received packet signal and a demodulator (DEMO) 9. The output of the RSSI 8 is sent to a carrier sense judgment device (CS) 10. When carrier sense is ON, the carrier sense judgment device (CS) 10 sends a demodulation start instruction to the DEMO 9.
As shown in FIG. 3, in the configuration of the receiver according to the prior art 2, a radio frequency signal is received by an antenna 1, and sent to a LNA 3 via a selector switch 2 to be amplified. The amplified signal and the output of a LO 5 are input in MIXERs 4 and 11 to perform frequency conversion directly to baseband.
For the baseband signal converted using such direct conversion method, quadrature detection (I/Q quadrature modulation/quasi-synchronous detection) is performed. In this case, an adjacent channel interference wave is suppressed by providing baseband I/Q sections with band fixed low-pass filters (LPF) 12 and 13 each having a cutoff frequency corresponding to one-channel modulation occupied band width (passband=one half of modulation occupied bandwidth) or more, respectively, so as to secure the channel selectivity characteristic (in the case of zero IF, LPF cutoff frequency=half or more than half of one channel).
The outputs from the LPFs 12 and 13 of the respective I/Q sections are connected to an RSSI 8 for carrier sensing the preamble of a received packet signal and a DEMO 9. The output of the RSSI 8 is sent to a CS 10. When carrier sense is ON, the CS 10 sends a demodulation start instruction to the DEMO 9.
Incidentally, the LPFs may be digital LPFs placed in digital baseband I/Q paths after IF sampling and digital synchronous detection process, or analog LPFs placed in analog I/Q baseband paths between analog quasi-synchronous detection and A/D conversion. Besides, both the digital LPF and analog LPF may be used to provide selectivity to the respective LPFs.
FIG. 4 is a diagram for explaining the reception operation of a conventional carrier sense multiple access system comprising a received signal strength indicator (RSSI) 8, a demodulator (DEMO) 9, and a carrier sense judgment device (CS) 10. As shown in FIG. 4, when the signal wave of the preamble of a received packet signal is input, the RSSI 8 detects the electric field strength of the received signal in faithful accordance with time waveform (step 100). The output of the RSSI undergoes averaging operation in the CS 10, and is compared with a carrier sense threshold value. When the output after the averaging operation is determined to be equal to or more than the preset threshold value corresponding to the minimum reception sensitivity level, the CS 10 recognizes that “carrier sense is ON” (step 101), and accordingly, sends a demodulation start instruction to the DEMO 9 (step 102). Having received the demodulation start instruction, the DEMO 9 initiates the demodulation process (step 103).